Alloy steel



Patented Nov. 7, 1950 ALLOY STEEL William Charles Clarke, J12, Dundalk, Md, as-

signor to Armco Steel Corporation, a corporation of Ohio No Drawing. Application October 23, 1946, Serial No. 705,245

- 5 Claims.

This invention relates in general to stainless steel especially to stainless steel articles suited to elevated temperature applications.

One of the objects of my invention is the provision of hot-workable austenitic stainless steel of high temperature strength.

Another object. is that of providing a practical and reliable method for producing strong, tough and durable austenitic stainless steel, illustratively in the form of wrought products, for service under load in hot, corrosive atmospheres.

A further object is the provisionof austenitic stainless steel having highly satisfactory and dependable resistance both to cree and stressrupture under load at elevated temperatures.

Other objects in part will be'obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, features of composition, and in the several steps and the relation of each of the same to one or more of the others, as described herein, and the scope of the application of which is represented in the following claims.

As conducive to a clearer understanding of certain features of my invention it may be noted at this point that the stainless steels in general, whether they be of the chromium-nickel grade or. straight-chromium grade, to some valuable extent possess the property of surface stability at high temperatures and are useful where resistance to corrosive attack in the presence of heat is of importance. Recently, however, demands for heat-resisting steels have widened considerably. The present day steel for high temperature duty is frequently called upon not only to serve with appreciable immunity from the attack of certain corrosive agents, but to possess great strength and resistance to creepan'cl rupture while under load at high temperatures. At times,v

the high temperature steels are turned to, either or both for their'corrosion resistance or for their mechanical properties. These diverse needs are not always fulfilled by stainless steel of specific composition; for example, two alloy steels of different composition may be equally effectiverin their resistance to oxidation at high temperatures, yet while heated may display widely different properties, with regard to strength at high temperatures, especially after being in use for a substantial period of time. The conventional austenitic chromium-nickel stainless steels are seriously limited in their physical properties athigh temperatures. They are subject to creep at elevated temperatures, that is the tendency toward plastic deformation, coupled with loss of strength tending toward rupture and unreliability under high stress. Engineering requirements have so far exceeded the properties available in the conventional austenitic chromium-nickel stainless steel that other alloys are being sought for high temperature duty.

In general, it can be said that the chromiumcontaining martensitic or ferritic stainless steels have a greater strength at low temperatures than do those of the austenitic grade. It might be expected that these properties would be retained at fairly high'temperatures. It nevertheless'is a fact, however, that the austenitic chromiumnickel stainless steels have a more favorable lattice structure for cohesion under high mechanical stress at temperatures above about 1000 F. thando the martensitic or ferritic stainless steels. Still, it is well recognized that the conventional austenitic steels, are unsatisfactory for meeting the exacting requirements of high temperature use, particularly where the steels are to be relied upon for withstanding high mechanical stress.

The known creep resistant and stress-rupture resistant alloys heretofore recognized as having high temperature properties are notoriously diflicult to work. They may have a Variety of properties which promise satisfactory performance at high temperatures yet do not respond favorably to working operations applied,

thereto for achievinghigh temperature products. The final products too frequently are found to have imperfections resulting from inferior working properties of the metal.

An outstanding object of my invention accordingly is the provision of high temperature austenitic chromium-nickel stainless steel wrought products and articles, which are sound and capable of resisting the damaging effects of corrosion and heat and of withstanding creep and rupture while subjected to high mechanical stress at elevated temperatures. I

Referring nOW more particularly to the practice of my invention, I achieve high temperature austenitic stainless steel by properly selecting and correlating as ingredients thereof the'elemerits chromium, nickel, iron, carbon and phosphorus and also copper, molybdeum, columbium,

' titanium andmanganese; the amounts of each in general ent the stress-rupture values and creep-resistance fall off; also the working qualities of the metal sufier. By discreet use of the'phosphorus constituent, the steel exhibits enhanced properties for resisting creep and stress-rupture at elevated temperatures. More specifically I find that stainless steel containing approximately 0.01% to r 0.15% carbon, 12% to 22% chromium, 10% to 21% nickel, from 0.05% to 0.50% phosphorus, 0.10% to 2% manganese, copper ranging between 2% and 4%, from 2% to 4% molybdenum, 0.15% I to 0.75% titanium, 0.20% to 1.10% columbium, and the remainder substantially all iron possesses excellent properties in use. Small amounts of sulphur not exceeding about 0.03 and not more than about 0.60% silicon, are preferred in the steel as compared with larger amounts of these elements.

The composition limits of my steel are regarded as being of a critical nature, for where they are departed from one or more of the desired qualities suffer. With a higher carbon content for example the working properties suffer and products of the metal are not so easily produced. Likewise, with a manganese content exceeding 2% there is a loss in hot-working properties. With any appreciable lowering of the phosphorus, copper and molybdenum contents the desired high temperature load-carrying characteristics of the steel suffer, and with appreciable increase workability disappears. Both the elements titanium and columbium in the amounts indicated further constitute a desired part of the alloy. In combination, and with proper heat treatment of the .steel, they enhance resistance to stress- I ploying heat treatment of the nature of an an-.

healing and precipitation treatment. In the annealing operation, I heat the steel to a sufiiciently high temperature and for long enough time to put at least part of the copper, titanium and columbium into solid solution. Where desired, advantage may be taken of the temperatures for annealing to hot work the steel. .A preferred temperature range for this purpose is that of about 2050 F. to 2250 F. The solubility of the ingredient columbium, I find increases with temperatures within the range just noted.

Following the heating for solubility, I quench the steel preferably to about room temperature, for example while using air, oil or water as the quenching medium. This annealed steel is substantially wholly austenitic. It is formable and machinable and, illustratively by these measures, is readily'made into desired high temperature I products and manufactures as for example into steam turbine blades, rotors, buckets, nozzles, turbo-superchanger parts, heat engine valves as for internal combustion engines, high temperature chemical equipment parts as those operating under high pressure, rivets, bolts and fasteners, and numerous other products which are intermittently or continuously subjected to either or both corrosive attack and high mechanical stress in use. The steel and products thereof are 4 strong, tough and durable at the high temperatures of use whether these temperatures be constantly high or prevail by virtue of intermittent cooling and reheating of the metal. 7

After the annealing and quenching operations, I re-heat the austenitic stainless steel, this time preferably to within an approximate temperature range of 1200 F. tol500 for precipitation treatment. Upon holding the metal at temperature for a suflicient period of time, titanium, columbium and copper precipitate in the matrix. The precipitates are in finely divided condition and are critically dispersed in the metal lattice along the atomic slip planes. They are believed to include intermetallic compounds of titanium and columbium. The copper comes out in fine form, or possibiliy as an intermetallic compound including titanium, columbium and nickel.

At the conclusion of the precipitation treatment, I quench the steel. The quenched metal has a fine grain structure and is further characterized by enhanced load carrying capacity and creep resistance in'view of atomic slip interference exerted by the precipitates. These precipitates remain uncoalesced between the atomic slip planes and effective against creep and stress rupture of the steel for extremely long periods of time at high temperatures.

In preferred composition my steel consists, in approximate percentages, of about 0.07% to 0.09% carbon, 16.50% to 17.50% chromium, from 13.25% to 14.25% nickel, on the order of 0.10% phosphorus, manganese ranging from 1.0% to 1.5%, from 2.85% to 3.35% copper, 2.60% to 3.10% molybdenum, titanium from 0.20% to 0.35%, 0.35% to 0.50% columbium, and the remainder substantially all iron. Only small amounts of silicon and sulphur preferably are present in the steel.

As illustrative of the practice of my invention, I provide austenitic chromium-nickel stainless steel of the preferred composition just noted, as by producing a melt of the same in a conventional electric steel making furnace of the'Heroult type in accordance with any one of several well-known melting processes having general applicability to the production of stainless steel such, for exam-' ple, as those processes generally described in the United States Letters Patent 1,925,182, issued to Alexander L. Feild on September 5, 1933, or

United States Letters Patent, 2,056,162, issued to 7 William B. Arness on October 6, 1936, or in the co-pending application of Donald L. Loveless, Se-

rial No. 628,673, filed November 14, 1945, now Patent No. 2,455,073, issued November 30, 1948. I work the austenitic steel as from ingots into bil-' lets by hot-rolling or forging at a temperature of about 2250 F. By forging, the billets then are roughly formed into .such products as blades and shafts for. gas turbines. Machining to size is achieved as desired. The fabrication of certain articles or products of the high temperature metal by welding with an oxy-acetvlene torch, or electric are means, employing welding rodspreferably of approximately the same analysis asthe' parent metal, a so is undertaken where desired.

It is advantageous, as after rough forging and machining my turbine parts,to subjectthe same to precipitation treatment. This illustratively is achieved by heating the steel parts in a conven-' tional heat treating furnace, say for about five hours at approximately 1200" F. The 'heat treated articles or products then are quenched as by cooling in air to an enhanced load-carrying condition.

The gas turbine equipment which I pro'd'uc'e is resistant to heat and corrosion. It is suited for withstanding the rigorous effects of high temperatures up to about 1500 F., or more, over long periods of service under stress without diificulty from grain growth or creep or from failure by fatigue or rupture. The stainless steel equipment has properties of high strength whether subjected to tension, compression, torsion or loads thoroughly practical advantages, are successfully achieved. It will be seen that the steel and products are tough, strong and durable, corrosion-resistant and heat-resistant and are well adapted to withstand high temperature duty over long periods of time and under variable conditions'of actual practical use. the method employed is instrumental to the achievement of products having superior high tending to shear the metal while hot and in use. temperature properties. The tur in q ipm n i r sistant o W rpi While the annealed and precipitation heat and resists harmfu Sca a d COITOSiVe treated products offer improved resistance to Secur a a y oxidizing a reducing ses creep and stress-rupture as compared with proda h temperatures ucts of the steel which have not been subjected ab I a II are providedbelow 1;0 illustrate to heat treatment, either the treated or untreated the exceptional stress-rupture and creep properprod t have u ef l high temperature properties of my austenitic stainless steels. The Table I t which may be availed upon in the light o identifies four steels, the first being steel A which immediate s, falls outside the scope. of the present invention It will also be appreciated that under certain but forms a basis for comparison with Steels B, '30 conditions of high temperature use of the steel, C and D having compositions in accordance with reliance may be had upon the prevailing high this invention. temperatures to achieve precipitation for en- 'IABLE I Composition (Per Cent) Steel 0 P Mn Cr Ni Mo Cu Ob Ti Fe A. 0.087 None 1.40 17.00 13.23 2 s0 3. 0 42 032 Balance B 0 100 0. 053 1. 50 10.00 13. 20 2.15 3.12 0. 47 0. 33 Do. 0. 0 00s 0. 007 1. 04 10.05 13. 22 2. 92 3.05 0. 0.20 Do. 11.--. 0.101 0.103 1.23 17.00 13.37 2. 05 a. 02 0.43 024 Do.

The steels in Table I, upon being heated at hancingthe load-carryingcapacityand resistance 2050 F. for one-half hour, water-quenched and t creep, re-heated at 1200 F. for five hours and aga n As many possible embodiments may be made of water-quenched were tested under like condimy invention and as many changes may be made tions, with results as reported in Table II. These in th embodiment hereinbefore set forth, it is results represent maximum endurance (in hours) to be understood that all matter described herein of str (1 S. i) applied under the high is to be interpreted as illustrative and not as a perature condition of 1200 F. They also enable t t a comparison of creep properties. Whenever the I l i decrease in the various maximum permissible 1 Austonitio chromium k l phosphorous Stresse as y be pp ny one of the given stainless steel of good hot-working properties and steels is relatively small with respect to time, the great Strength and toughness t high temperagreater is the resist n e to r ptures containing 12% to 22% chromium, 10% to TABLE II 21% nickel, 0.05% to 0.50% phosphorus, from 0.01% to 0.15% carbon, 0.10% to 2% manganese Stress-endurance 2% to 4% molybdenum and 2% to 4% copper, from 0.15% to 0.75% titanium, 0.20% to 1.10% Maximum stress L) Enduredfor columbium, and the remainder substantially all Steel iron.

100 hrs. 1,000 hrs. 10,000 hrs. 100,000 hrs. 2 Austenitic chromium nickel phosphorus stainless steel of good hot-working properties and great strength and toughness at high tempera- 45: 000 42000 411000 401000 tures containing 16.5% to 17.5% chromium, from 47,000 41000 13.25% to 14.25% nickel, about 0.10% phosphorus, from 0.07% to 0.09% carbon, 1.0% to 1.5% 'Extrapolated manganese, 2.6% to 3.1 molybdenum and 2.85% My phosphorus-containing.stainless steel has, to 325 copper, from 020% t 035% tit i by virtue of the particular combination of ele- 0.35% to 5 columbium, and t remainder ments therein, the remarkable ability to be substantially n iron Worked and fabricated into pmducts having high" 3. Austenitic chromium-nickel stainless steel temperature properties which are readily en- M having good hot working properties and great hanced, if desired, by precipitation slip-interference treatment of the metal as during the product manufacturing operations. The comparative ease of working the high-temperature metal, as by means of hot-forming operations, contributes for simplicity and directness in achieving desired strength and toughness at high temperatures and containing 12% to 22% chromium, 10% to 21% nickel, from 0.05% to 0.50% phosphorus, 0.01% to 0.15% carbon, 0.10% to 2% manganese, from 2% to 4% each of molybdenum and copper, 0.15% to 0.75% titanium, 0.20% to 1.10% columbium, said copper, titanium and columbium being at least partly precipitated in critically dispersed form for slip-interference, and the remainder substantially all iron.

It will also be seen that 7 '4; 'Austenitic chromium-nickel stainless steel having good hot-working properties and great strengthand toughness at high temperatures and containing 16.5% to 17.5% chromium, 13.25% to 14.25% nickel, about 0.10% phosphorus, from 21% nickel, from 0.05% to 0.50% phosphorus,,

0.01% to 0.15% carbon, 0.10% to 2% manganese, from 2% to 4% each of copper and molybdenum, 0.15% to 0.75% titanium, from 0.20% to 1.10% columbium, and the remainder substantially all iron, said copper, titanium and columbium being at least partially precipitated in critically dis: persed form for slip-interference.

WILLIAM CHARLES CLARKE, JR. 1

REFERENCES CITED The following references are of record in the file of this patent;

UNITED STATES PATENTS Number Name Date 2,174,025 Wise et al. Sept. 26, 1939 2,401,580 Mohling June 4, 1946 2,402,814 Hatfield June 25, 1946 2,447,897 Clarke Aug. 24, 1948 OTHER REFERENCES Steel and Its Heat Treatment, vol. II, 4th Edition, page 364. Edited by Bullens, published in 1939 by John Wiley and Sons, New York.

Metals and Alloys, February 1937, pages 53 to 58. 

1. AUSTENITIC CHROMIUM - NICKEL - PHOSPHOROUS STAINLESS STEEL OF GOOD HOT-WORKING PROPERTIES AND GREAT STRENGTH AND TOUGHNESS AT HIGH TEMPERATURES CONTAINING 12% TO 22% CHROMIUM, 10% TO 21% NICKEL, 0.05% TO 0.50% PHOSPHORUS, FROM 0.01% TO 0.15% CARBON, 0.10% TO 2% MANGANESE 2% TO 4% MOLYBDENUM AND 2% TO 4% COPPER, FROM 0.15% TO 0.75% TITANIUM, 0.20% TO 1.10% COLUMBIUM, AND THE REMAINDER SUBSTANTIALLY ALL IRON. 